Aspects of this disclosure are generally related to optical fiber devices, and more particularly to tuning optical fiber gratings. Optical fiber gratings have many applications and are widely used in fiber optic communication systems, fiber optic sensors and fiber lasers to selectively control the wavelength of light propagating in an optical fiber. A typical fiber grating includes a length of optical fiber in which a section of the fiber core has been modified to include a plurality of periodic perturbations in effective refractive index along the length of the fiber.
Fiber Bragg Gratings (FBGs) and Long Period Fiber Gratings (LPFGs) are types of fiber gratings which are distinguished by differences in the periodic spacing of the effective refractive index perturbation along the length of the fiber grating. FBGs reflect particular wavelengths of light in the fiber and transmit all others. The wavelength λB reflected by FBG can be characterized by λB=2nΛB, known as the Bragg condition, or Bragg resonance wavelength, where λB is the center wavelength of reflected light from the fiber grating, n is the effective refractive index of the fiber core, and ΛB is the period of refractive index perturbation in the fiber. FBGs generally have a narrow wavelength band spectral response and may be used as a narrow band filter or reflective mirror in an optical fiber system. LPFGs have a physical configuration similar to that of FBGs, but the grating period ΛL of typical LPFGs is much longer than the grating period ΛB of typical FBGs. More particularly, ΛL may be from 200 to 2000 times longer than ΛB. An LPFG operates by coupling the fundamental mode in the fiber core to the cladding modes of the fiber. The excited cladding modes are then attenuated, resulting in the appearance of resonance loss in the transmission spectrum. Consequently, in contrast to FBGs, LPFGs do not produce reflected light. Phase matching between the fundamental mode and cladding modes at wavelength λmL can be expressed as: λmL=(ncore−nclm) ΛL; where, ncore is the effective refractive index of the fundamental mode and nclm is the effective refractive index of the mth cladding mode, and ΛL is the period of the LPFG. Since several cladding modes can satisfy this condition, each one is at different center wavelength λmL. Consequently, the transmission spectrum of the LPFG exhibits a series of transmission loss peaks along the spectrum distribution.
The structure of the modified fiber section of fiber gratings is associated with variations of amplitude or period of effective refractive index perturbation along the length of fiber. The optical properties (or spectral response) of a fiber grating are a function of the profile of refractive index perturbation along the length of the fiber grating section. The grating period in the fiber grating section can be uniform or graded, and either localized or distributed in a superstructure along the length of the fiber. Examples of various different structures and their spectral responses are described in: Turan Erdogan, “Fiber Grating Spectra,” in Journal of Lightwave Technology, Vol. 15, No. 8, 1277-1294, August 1997; X. Liu, “Design of superstructure fiber Bragg gratings with a Fourier analysis technique and its applications to multiple ultra narrow transmission gratings,” in Optical Engineering, Vol. 47, No. 11, 115001-7, November 2008; Maxim A. Bolshtyansky, “Grating-based dispersion compensation devices” in U.S. Pat. No. 6,710,916 (describing the chirped superstructure fiber gratings for application in optical amplifiers and optical network); Ben Eggleton et al., “Broadband grating” in U.S. Pat. No. 6,081,640 (describing several fiber grating structures and application); and Hojoon Lee et al., “Purely Phase-Sampled Fiber Bragg Gratings for Broad-Band Dispersion and Dispersion Slope Compensation,” in IEEE Photonics Technology Letters, Vol. 15, No. 8, 1091-1093, August 2003.
In general, known types of FBG structures according to refractive index perturbation profiles include uniform FBGs, apodized FBGs, chirped FBGs, phase shifted FBGs, tilted FBGs, superstructure FBGs and LPFGs. Uniform FBGs may be characterized by uniformly distributed refractive index perturbations along the length of the fiber. The spectral response of uniform FBGs includes narrow band reflection and notch transmission. In order to improve side-lobe suppression and maintain reflectivity and bandwidth, the apodization of uniform FBG is normally adopted, in which the amplitude of refractive index perturbation profile of the FBG is ramped up and down along the grating. Typical applications of uniform or apodized FBGs include use as an optical filter in communication systems and as a feedback reflector in a fiber laser system. Chirped FBGs may be characterized by a refractive index perturbation profile having a monotonically varying grating period or linear variation of the grating period along the fiber, e.g., a grating period which increases between each perturbation in series. Chirped FBGs provide broadband spectral reflection in their spectral response. Typical applications include use as dispersion compensation devices in optical communication systems. Phase shifted FBGs may be characterized by changed period spacing at certain point in the refractive index perturbation profile relative to uniform FBGs. A phase shift jump is introduced at the insertion point of the refractive index profile. Phase shifted FBGs introduce a very narrow transmission band within their reflection bandwidth. The phase jump opens up a bandgap within the reflection bandwidth, creating a narrow transmission band. Typical applications include use in DFB (distributed feedback) fiber lasers to generate stable single frequency laser operation. In principle, multiple phase shifts can be introduced into a FBG at one or more locations along the FBG, which can generate multiple narrow transmission bands within the reflection bandwidth of the FBG. Tilted FBGs may be characterized by refractive index perturbation along the length of the fiber being set at an angle relative to the optical axis. Tilted FBGs can couple core mode to cladding mode and radiation mode. The transmission spectrum of tilted FBGs exhibits many resonance peaks. Superstructure FBGs may be characterized by a varying refractive-index perturbation profile distributed in a superstructure along the length of the fiber grating. Superstructure FBGs may have a relatively complex refractive-index perturbation profile which can vary via amplitude, period or both along the length of fiber, e.g., exhibiting multiple nested patterns of perturbations. One example of a superstructure FBG is the so-called “Sampled FBG,” which is generated by a periodic amplitude or phase modulation in the refractive index perturbation profile of the FBG along the length of the fiber grating. The resultant spectral response shows multiple narrow reflection channels. The separation and bandwidth of the spectral channels are a function of the sampled period of the refractive index perturbation profile modulation. Furthermore, a full complex superstructure FBG may include a complex refractive-index profile modulation and multiple phase shifts along the fiber grating. Phase shifted FBGs and chirped FBGs may be viewed as examples of superstructure FBGs.
A LPFG may be characterized by a grating period ΛL which is much longer than an FBG grating period ΛB. In contrast to FBGs, LPFGs exhibit a spectral response having a series of transmission loss peaks along the transmission spectrum distribution, and do not produce reflection light in the fiber. Typical applications include band-rejection filters and fiber sensors. Similar to FBGs, LPFGs can have a phase shifted structure or a superstructure grating type.
The spectral response of a fiber grating can be affected by strain or temperature applied to the fiber grating. For example, the center wavelength (or resonance wavelength), λB=2nΛB, can be changed by changing the effective refractive index n of the fiber core or the period ΛB of the fiber grating by applying strain or a temperature delta to the grating. For a given strain εz, the center wavelength shift of the FBG is ΔλB=λB (1−p)εz, where p is an effective strain-optic constant. For a given temperature change ΔT, the center wavelength shift is Δ λB=λB (αA+αB) ΔT, where αA is the thermal expansion coefficient of the fiber and αB represents the thermo-optic coefficient. For a typical FBG with center wavelength at 1550 nm, the strain induced wavelength shift is about 1.2 pm/με, and the temperature change induced wavelength shift is around 12.8 pm/° C. This effects are the working principle of the FBG based fiber sensors. These physical characteristics can be also used to tune the center wavelength of a FBG, i.e., by applying controlled strain or heat to the FBG to obtain desired center wavelength.
A variety of specific techniques for tuning the spectral response of FBGs with strain and temperature delta are known. One technique, described by Morey, et al. in U.S. Pat. No. 5,469,520, entitled “Compression Tuned Fiber Grating,” is to put FBGs inside sliding ferrules and place the ferrules in a mechanical structure to guide and confine the fiber. Another technique, described by Fernald et al. in U.S. Pat. Nos. 6,229,827 and 6,363,089 entitled “Compression-Tuned Bragg Grating and Laser,” fuses the FBG in a glass capillary tube. Another technique, described by Long in U.S. Pat. No. 6,360,042, entitled “Tunable optical fiber gratings device,” is to bond the FBG on a cantilever beam. The beam can then be bent in different directions, resulting in application of compressive or tensile strain to the FBG. In U.S. Pat. No. 7,801,403, F. Luo et al. describe a fiber grating tuning device which uses corrugated deformable slides to apply strain to tune a fiber grating. All of these techniques are designed for tuning center wavelength shift of a FBG. The tuning strain is applied to the entire fiber grating structure, which introduces a complete spectrum shift. Further, the techniques are limited to tuning the FBG from one structure to another structure in order to obtain a different spectral response, e.g., tuning a uniform FBG into a phase shifted FBG or a superstructure FBG.
A variety of other tuning techniques are known, including the techniques described below.
Feng et al. in U.S. Pat. No. 6,453,095, entitled “Tuning of optical dispersion by using a tunable fiber bragg grating,” describe a technique for tuning chirped and sampled nonlinearly-chirped fiber gratings by using a piezoelectric element to stretch the FBG.
Moo-Youn Park in U.S. Pat. No. 6,246,814, entitled “Tunable chirped fiber grating device and method for forming chirped fiber grating” describe a tunable chirped grating device in which a piezoelectric element is bonded to a FBG for changing of the perturbation spacing. Piezoelectric elements typically require a high voltage drive and have limited driven displacement, thereby limiting tuning capability.
Michel J. F. Digonnet et al. in U.S. Pat. No. 6,282,341, entitled “Tunable, mechanically induced long-period fiber grating with enhanced polarizing characteristics,” describe a mechanical stress induced LPFG in which the LPFG is generated by applying lateral stress on an optical fiber along the length of the fiber periodically.
Yize Huang, “Tunable superstructure fiber Bragg grating with chirp-distribution modulation based on the effect of external stress,” Optics Letters, Vol. 37, No. 18, 3918-3920, 2012, describes a superstructured FBG with multichannel reflection peaks formed by using a piezoelectric actuator to apply lateral stress along an FBG periodically. Applying lateral stress to an optical fiber will introduce birefringence in the fiber which causes additional polarization problems.
Ximin Zhao in U.S. Pat. No. 6,721,478, entitled “Dynamic fiber bragg grating,” describes a technique in which a FBG is generated by using heating elements arranged relative to an optical fiber to form periodic “hot spots” along the length of fiber. Theoretically, the generated fiber grating can be tuned thermally. However, optical fibers have a small temperature coefficient and generating periodic high temperature spots along a fiber in the scale of the FBG period ΛB (typically, ΛB˜0.53 um at 1550 nm wavelength band) would be difficult or impractical.